(1) Field of the Invention
The present invention regards a loading buoy for loading and unloading liquids and gases in open sea.
(2) Description of Related Art
Several buoys of this type are known. The prior art buoys support one end of one or more risers that extend from storage tanks on the seabed, e.g. at a nearby production facility. The buoy is moored to the seabed and floats a distance below the surface of the sea, due to a positive buoyancy. When used for loading or unloading, the buoy is pulled up into a pick-up space in a vessel and held there. A swivel connected to loading and unloading lines aboard the vessel is coupled to the top of the buoy and connected to the tops of the risers from the buoy. Loading or unloading can then commence.
The buoy comprises an outer and an inner part which are interrotatable, the outer part being kept rotationally fixed relative to the vessel, and the inner part being substantially rotationally fixed relative to the seabed, by way of the mooring. Thus the vessel can rotate under the influence of wind and current without affecting the loading or unloading process.
A common feature of known solutions such as described in inter alia U.S. Pat. Nos. 4,490,121, 4,604,961, NO 176 131, NO 175 419, NO 175 420 and NO 175 421 is that of the buoy being moored to the seabed. Mooring the buoy may seem natural, as it is desired that the buoy stay substantially stationary. The mooring also serves as mooring for the vessel when connected to the buoy. This allows the vessel to weather a storm without there being any risk of it coming adrift and without wind and current affecting the loading and unloading operation.
However, mooring the buoy also carries drawbacks. The mooring requires the buoy to absorb all the forces exerted by the mooring line. These forces can get very large when there are heavy currents affecting a moored vessel. Consequently the buoy must be dimensioned to take up these forces. Heavy dimensioning means a high steel weight. A high steel weight requires the buoy to have a relatively large buoyancy chamber in order to float at the correct depth. This means that the buoy will be quite large. A large buoy requires a large pick-up space in the vessel. A large pick-up space has implications for the support structure of the vessel and therefore can not be placed just anywhere in the vessel. Thus the pick-up space has been located at the bow, which is already built to take up large stresses and strains.
However, all the above factors will impose certain limitations on the design of both the buoy and the vessel. The present invention aims to eliminate or at least bring about a substantial reduction in these limitations.